Estimate breaks with previous research, prompting skepticism.

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Extracting information from the ice-covered continent of Antarctica isn’t easy. You have to go a long way just to ask, and the region isn’t exactly known for its hospitality. And yet, the answers we seek are important ones—if any significant portion of the ice sheets melt as the climate warms, the sea will advance on coastal areas around the world.

Antarctica’s coastal outlet glaciers are on the front lines of climate change, and so may get visits from curious scientists. But the expanse of the continental interior is even tougher to study. Satellites are absolutely indispensable for efforts to quantify the big picture answers there, like how much Antarctica’s icy burden is changing.

A number of satellite techniques are being tried, from precise measurements of the surface elevation of the ice using radar or lasers, to sensitive measurements of Earth’s gravitational pull to detect mass changes, to simple input-output models based on snowfall data and tracking the velocity of the ice that flows into the ocean.

Different techniques, along with refinements of analyses over time, have produced a range of estimates of Antarctic mass change. While there’s no question that many key outlet glaciers, which drain the ice sheet, are shrinking, there are other factors to consider. Is a warming ocean and atmosphere bringing more snow inland to replenish the ice sheets? Is the interior of the ice sheet deflating like a squeezed tube of toothpaste, complicating analysis of changing outlet glaciers?

Putting it all together, published estimates of total mass change have ranged from slight gains to large losses, all with pretty significant error bars.

A new study published in the Journal of Glaciology by a team led by NASA researcher Jay Zwally produces a surprising and controversial new estimate of large mass gains. Zwally's team has been working on satellite measurements of ice height for some time, but this estimate is significantly different from even their own previous work.

The height of the ice

The new paper is dense because these analyses are extremely complicated. The starting point is the ice height data collected by the European Remote-Sensing Satellite between 1992 and 2001, and by NASA’s ICESat between 2003 and 2008.

Turning that data into estimates of ice mass change involves many steps. The measurements have to be carefully calibrated, with any change in each satellite’s orbit over time precisely accounted for. A host of other calculations have to be made, including small surface elevation changes due to the compaction of the slowly buried snow (which requires snowfall data) and the rising of the bedrock beneath the ice sheet, which is still rebounding from the much larger weight of ice it bore during the last ice age. The measurements of the earlier satellite even have to be corrected for the depth the radar signal penetrates into the snow, which varies.

The researchers have put a lot of work into these calculations, and the end result is an estimated Antarctic ice mass gain of 112 ± 61 gigatons per year between 1992 and 2001, and 82 ± 25 gigatons per year between 2003 and 2008. (Although some regional changes between the two time periods are statistically significant, the researchers note that the two ranges overlap.) For comparison, a major 2012 study estimated a loss of 71 ± 53 gigatons per year between 1992 and 2011.

Rather than contributing to rising sea levels, the new study estimates that Antarctica is counteracting sea level rise by roughly 0.2 millimeters per year.

The researchers explain their result in terms of inland snowfall. Since the warmth arrived at the end of the last ice age, snowfall has been greater. Because the frigid inland ice flows so slowly, it still has not reached equilibrium with this new surplus, and is thickening as a result. Accelerating ice loss in West Antarctica shows up in their estimates just as it does in others, but it is overshadowed by this inland thickening.

To be clear, the researchers are not making the claim that climate change is somehow not a problem, or that sea levels aren’t rising. But they do see this thickening as a “buffer” against mass loss. If outlet glaciers continue their accelerating shrinkage, and no increase in snowfall counteracts it, they say Antarctica would start contributing to sea level rise in about 20 years. If snowfall increases further, it would take longer.

Imprecision

The paper is attracting a lot of criticism from other scientists, however. Eric Rignot, who has worked on similar Antarctic studies, has been fielding a lot of questions about this one and shared his comments with Ars. “I am afraid I have some rather harsh words about it,” he wrote. Rignot said the data just aren’t precise enough to support the paper's conclusions.

“Zwally's group is the only one pushing its interpretation beyond the realm of the inherent uncertainty of the data. Accumulation of snow in East Antarctica is 10 centimeters water equivalent per year. To detect changes in accumulation of 136 gigatons (with no error bar), or 10 percent, Zwally et al. needs to detect changes of the order of 1 centimeter. Current technology such as ICESat cannot detect any change smaller than 20 centimeters. Radar altimetry (used for the earlier period) is closer to 40-50 centimeter noise, but nobody really knows. There is no way to detect changes in East Antarctic accumulation at the 10 percent level, not even 50 percent level, with those kinds of error bars,” Rignot wrote.

He continued, “The Zwally group’s findings are at odds with all other independent methods: re-analysis, gravity measurements, mass budget method, and other groups using the same data.”

Andrew Shepherd, another researcher who has used satellite data to study Antarctica’s ice, shared similar impressions with Ars. "Zwally and his team have tried to account for snowfall, which masks changes in the thickness of the polar ice sheets,” Shepherd wrote. “It's right to attempt this, but in places where nothing much happens—like the interior of Antarctica, which is a vast a desert—it's really quite difficult to be sure that snowfall can be simulated with enough precision to detect ice imbalance."

Shepherd sounded confident that the differences among the estimates won't last for long. "Fortunately we now have many different ways to examine Earth's ice sheets—from space and on foot," he said, "and I'm confident that we can get to the bottom of this contradiction by taking everything into account."

Since there are a lot of important variables in the calculations—Antarctica’s rebounding bedrock is a particularly significant one—there are lots of things for scientists to argue about. In fact, Zwally and his co-authors argue that their preferred values for some of those variables can bring other analyses closer to their new numbers. Future research should hopefully provide some clarity as to whether this study is more than a quirky outlier. For now, it is certainly true that it’s an outlier.